Activatable temperature indicator with time delay

Abstract

Disclosed herein are delayed reaction threshold temperature indicators and methods of making and activating the same, the delayed reaction threshold temperature indicators including a first substrate, first and second layers, and a housing secured to the substrate. The first layer includes a first reactant and an optional meltable polymer, and the second layer includes a meltable polymer and a second reactant. The meltable polymer is configured to keep the first and second reactants from interacting with each other. When exposed to temperatures at and/or above a desired threshold for a period of time, the meltable polymer melts and allows the first and second reactants to come into contact with each other, thereby producing a visual change in appearance.

The invention claimed is: 
     
       1. A delayed reaction threshold temperature indicator with a predetermined exposure time threshold comprising:
 a first substrate 
 a first layer further comprising a first reactant, the first layer positioned on top of the substrate; 
 a second layer further comprising a meltable polymer and a second reactant, the second layer positioned on top of the first layer; 
 a housing secured to the substrate, the housing including a viewing window through which the first and second layers are visible; 
 wherein exposure to a temperature at and/or above the melting point of the meltable polymer allows the first reactant and/or second reactant to migrate toward each other; 
 wherein, when exposed to a temperature at the melting point of the meltable polymer, the first and second reactants require at least a period of exposure time to the temperature at the melting point that is greater than the predetermined exposure time threshold in order to migrate into contact with each other; 
 wherein the first and second reactants produce a visual change in appearance after they come into contact with each other, even if the temperature returns to below the melting point of the meltable polymer after they come into contact. 
 
     
     
       2. The indicator of  claim 1 , wherein the first substrate includes a background comprising a pattern, an indicia, or a color darker than an initial color of the first and second reactants prior to their contacting each other. 
     
     
       3. The indicator of  claim 1  wherein the predetermined exposure time threshold is at least 5 minutes and less than or equal to 60 minutes. 
     
     
       4. The indicator of  claim 1 , wherein the meltable polymer resolidifies rapidly when exposed to a temperature below the melting point of the meltable polymer, thereby inhibiting the migration of the first and/or second reactants through the first and/or second layers, and preventing the visual change of appearance if the first and second reactants have not yet contacted each other. 
     
     
       5. The indicator of  claim 1 , wherein the first reactant is bromothymol blue or resorcinol. 
     
     
       6. The indicator of  claim 1 , wherein the second reactant is butylamine or 5-(hydroxymethyl)furfural (HMF). 
     
     
       7. The indicator of  claim 1 , wherein the first and/or second layer include an additive or catalyst configured to affect the reaction between the first and second reactants. 
     
     
       8. The indicator of  claim 1 , further comprising a reference area surrounding or adjacent to the viewing window. 
     
     
       9. A method of activating the indicator of  claim 1 , the method comprising:
 applying the first layer to a first face of the first substrate so that the first layer is adhered to the first substrate; 
 applying the second layer to a first face of the housing so that the second layer is adhered to the housing; and 
 affixing the housing to the first substrate so as to bring the first layer into contact with the second layer. 
 
     
     
       10. The indicator of  claim 1 , wherein the predetermined exposure time threshold is at least 10 minutes. 
     
     
       11. The indicator of  claim 1 , wherein the predetermined exposure time threshold is at least about 30 minutes. 
     
     
       12. The indicator of  claim 1 , wherein the visual change in appearance is a change in color. 
     
     
       13. The indicator of  claim 12 , wherein the change in color is a darkening of the first and second layers visible through the viewing window. 
     
     
       14. A combination indicator comprising;
 the indicator of  claim 1 , and 
 at least one of a freeze indicator, a second temperature threshold exposure indicator, and a cumulative heat exposure indicator. 
 
     
     
       15. The combination indicator of  claim 14 , wherein the second threshold indicator and/or the cumulative indicator is configured to monitor at least one of a change in or a period of exposure to temperature, pH, humidity, or radiation. 
     
     
       16. The indicator of  claim 1 , affixed to a product, a container, or a product packaging. 
     
     
       17. The indicator or combination indicator of  claim 16 , wherein the product, container, or product packaging contains a perishable substance. 
     
     
       18. The indicator of  claim 1 , further comprising a barrier layer between the first and second layers. 
     
     
       19. The indicator of  claim 18 , wherein the barrier layer is permeable to at least one of the meltable polymer, the first reactant, and the second reactant when the meltable polymer is at least partially liquid or at least partially amorphous. 
     
     
       20. The indicator of  claim 18 , wherein the barrier layer comprises a meltable material. 
     
     
       21. The indicator of  claim 20 , wherein the meltable material comprises the meltable polymer. 
     
     
       22. A method of activating the indicator of  claim 20 , the method comprising:
 applying the first layer to a first face of the first substrate so that the first layer is adhered to the first substrate; 
 applying the barrier layer to the first layer; 
 applying the second layer to a first face of the housing so that the second layer is adhered to the housing; and 
 affixing the housing to the first substrate so as to bring the first layer into contact with the barrier layer. 
 
     
     
       23. A method of making a delayed reaction threshold temperature indicator, the method comprising:
 applying a first mixture to a first substrate to create a first indicator element, the first mixture comprising a first reactant; 
 applying a second mixture to a second substrate to create a second indicator element, the second mixture comprising a meltable polymer and a second reactant; and 
 securing the first and second indicator elements to each other so that at least part of the second mixture is brought into contact with the first mixture and so that first and second mixtures are enclosed by the first and second substrates; 
 wherein melting of the meltable polymer allows the first and second reactants to migrate into contact each other only after the indicator has been exposed to a temperature above at or above a predetermined temperature threshold for at least a predetermined time period, to produce a visual change in appearance.